1. Field of the Invention
This invention relates to a backlighting system for illuminating a liquid crystal display as used in a display unit of a portable computer, portable or car TV receiver or the like.
2. Background Art
FIGS. 5 and 6 show an example of a backlighting apparatus 90 of the prior art. A light guiding plate 91 is formed such that a main body 91a is substantially rectangular, in shape, and is from an achromatic transparent resin such as an acrylic resin and fitted with tongue-shaped attaching feet 91b at a plurality of, positions of the periphery of the main body 91a.
A frame 92 is substantially U-shaped to surround the light guiding plate 91 from three directions and formed of a suitable white opaque resin member. Receiving portions 92a are shaped like a recess to be fitted onto the attaching feet 91b, and are formed in the frame 92 at positions corresponding to the attaching feet 91b.
The light guiding plate 91 and the frame 92 are connected by screws 93 to form into a single unit. In this integrated state, as shown in FIG. 6, a space D is formed between an end surface 91c of the light guiding plate 91 and an inner end surface 92b of the frame 92 so that they face each other with an air layer interposed therebetween.
FIG. 7 shows a state where the light guiding plate 91 and the frame 92 are closely connected with each other. In this state, when light L1 passes through the light guiding plate 91 and reaches the end surface 91c, irregular reflection occurs on the inner end surface 92b of the frame 92 that is in close contact with the end surface 91c. The radiation of the irregularly reflected light L2 causes such nonuniform illumination that a portion near the end surface 91c of the light guiding plate 91 is brighter than other portions, thus deteriorating the performance of the backlighting apparatus 90.
A target backlighting apparatus 90 is obtained by attaching a diffusion sheet 94 formed of a polyester film to the assembly of the light guiding plate 91 and the frame 92, the frame 92, or the light guiding plate 91 with a double-coated adhesive tape 95 at the entire periphery or two opposite sides thereof to cover the illuminated surface 91d of the light guiding plate 91.
However, the backlighting apparatus 90 of the prior art as described above has the following problems. Firstly, since the light guiding plate 91 and the frame 92 are connected with each other by the screws 93 to be integrated into a single unit, the light guiding plate 91 must have at least a thickness as large as the total length of the screw 93. Accordingly, the backlighting apparatus 90 becomes bulky, thereby increasing the size of an apparatus employing it such as a portable computer.
Secondly, since the light guiding plate 91 is partially attached to the frame 92 via the attaching feet 91b, attaching strength is insufficient, which may cause the attaching feet 91b to be broken by stress generated by a fall or vibration during the transportation of the apparatus. As a result, the light guiding plate 91 may be separated from the frame 92.
Further, since the diffusion sheet 94 is attached to the frame 92 at the entire periphery thereof, the difference of thermal expansion coefficients therebetween may cause diffusion sheet 94 having low strength to become curved as shown in FIG. 6, thereby causing nonuniform illumination. Solutions to these problems have been awaited.